1. Field of the Invention
This invention relates to a steerable or directionable transponders (e.g. antennae or emitters), and especially but not exclusively to interrogator systems incorporating such transponders, and to vehicles incorporating them.
2. Discussion of Prior Art
There are times when the user (or a control processor) of electromagnetic detecting apparatus needs to look in two different directions, or when two users of a detection system each independently want to look in a different direction (for example when two people both want to have, E.M. signals analysed that come from two different directions). Sometimes signals from both directions, or fields of view, need to be analysed at substantially the same time. The immediate answer to this need is to provide two signal capture systemsxe2x80x94for example two antennae, each under the control of a different person. This increases the cost of the overall detection system.
A second solution is to mount the signal capturer (e.g. antennae or camera) on a turntable or gimbal mechanism so that it can be rotated to look in the two directions at different times and ensure that the turntable can be moved fast enough to provided the required response time for both users (or the system) to see their desired field of view substantially simultaneously. This is difficult if a fast turning time (e.g. less than 1 second, and especially if  less than 200 ms) is required with a substantial mass of antennae. The motor drives needed will be large and expensive, and furthermore gimbal arrangements are susceptible to mechanical failure or damage due to exposed moving parts. A gimbal also requires a large volume.
Phased arrays could in theory be used to steer the signal capturer/antennae, however at high frequencies (e.g. GHz-range) the technology is not yet mature enough to be practical or affordable, especially for the case of broad-band signals.
One area where it is believed that a steerable antennae or interrogator would be useful is in the field of vehicles. For example it is known to have radar-controlled systems on cars which prevent a car from coming too close, whilst travelling, to the vehicle in front. In order to look at a sufficiently wide field of view using cheaper, practicable, equipment it may be necessary to have more than one antenna/detector. By having a steerable antenna it is possible to provide fewer antennae and yet still look across a wide enough field of view. It may be possible to provide only one antenna on the vehicle.
It may be desirable not only to know that there is a vehicle in front, but also to identify it as being one of a known kind. For example road trains and tram convoys have been proposed in which a trailing vehicle follows a lead vehicle automatically at a controlled distance and speed, and it may be desirable for the trailing vehicle to identify the lead vehicle, for example by interrogating a transponder, or by processing some other identifying signal from the lead vehicle. It may also be desirable for the signal from the lead vehicle to contain additional data beyond that it is there, for example, its identity, what it is carrying, its braking distance, amount of fuel it has on board etc.
It is an aim of one embodiment of the present invention to enable an antenna to be steerable at a speed of the order of 100 ms or so (when operating at GHz frequencies or above).
According to a first aspect the invention comprises a steerable antenna or scanner assembly having an antenna and a steering reflector, the steering reflector being adapted to be controlled by control means.
Thus the steering reflector is moved, and not the entire assembly. This enables the lower mass reflector to be moved faster. Also, since it is a reflector it can be moved through only half of the angle the entire assembly would have to be turned (if the movement is angular movement). Preferably the antenna is of the order of 50 cm2 or 100 cm2 at least in area. It may be larger or smaller, depending upon the desired resolution, and upon the wavelength used.
The steering reflector is preferably capable of being moved between a first position in which the assembly has a first field of view and a second position in which the assembly has a second field of view in a time of about half-a-second or less. Most preferably the steering time is not more than about 0.2 seconds, 0.1 second, or less, possibly about 10 ms. The steerable reflector may be moved in the steering time through an angle that is at least 10xc2x0, or 20xc2x0, or 40xc2x0, or 60xc2x0, or 90xc2x0, and is preferably movable in that time through at least about 90xc2x0.
The assembly is preferably adapted to detect or emit electromagnetic radiation, preferably of millimeter wavelengths, or possibly longer or shorter wavelengths. A wavelength of 36-40 GHz, or about 77 GHz, or about 95 GHz may be chosen.
The assembly may be housed in a protective housing, which may be of the order of 5-20 cm or more wide and may be about 10-20 cm or more tall, and may be about 1-20 cm deep.
The antenna is preferably stationary (relative to a housing or mounting frame), with the steering reflector being adapted for movement.
The assembly may direct incident radiation from a chosen direction to a detector via at least one reflection of the incident radiation. This enables it to be compact. Incident radiation from the chosen direction may be directed to the detector via at least two reflections of the incident radiation within the assembly, or with only, or no more than two reflections of the incident radiation within the assembly.
There maybe a selective reflector which in use transmits at least a substantial part of an incident wave under certain conditions and which reflects at least a substantial part of an incident wave under other conditions, and in which the arrangement is such that radiation reaching a detector has been both reflected from the selective reflector and transmitted through the selective reflector. Substantially all incident radiation that encounters the selective reflector in a particular wavefront may be transmitted, or substantially all may be reflected. The steering reflector may comprise the selective reflector.
A polarisation changer maybe provided to effect a change of polarisation of radiation. The polarisation changer may provide in use the change of polarisation between the selective reflection or transmission of the incident radiation by the selective reflector and may transmit or reflect the radiation dependent upon the polarisation state of the radiation incident upon it. The assembly may be adapted to reflect off the steering reflector radiation incident upon the assembly from a chosen direction. The antenna may be a focusing antenna, and the antenna may be adapted to focus the polarity-altered radiation reflected off the steering reflector onto a detector. The polarisation changer is preferably provided in the path of the radiation between its reflection from the steering reflection and the focusing antenna, the arrangement being such that the radiation reflected off the steering reflector has its polarity altered before re-impinging upon the steering reflector and passing through the steering reflector to reach the detector.
The antenna may be arranged relative to the polarisation changer such that the radiation incident upon the assembly from a chosen direction passes through the antenna before encountering the polarisation changer.
The antenna and/or steering reflector may comprise substantially parallel lines of material. The polarisation changer may rotate the polarity of a linearly polarised wavefront by substantially 45xc2x0 as the wavefront passes through it. The steering reflector may be movable angularly about an axis.
A circular polarisation to linear polarisation converter may be provided. There are certain advantages to communicating over distances using circularly polarised radiation. The circular polarisation to linear polarisation converter may be provided in front of the antenna, so that a wavefront incident from a chosen direction is incident upon the converter before it is incident upon the antenna.
Circular to linear polarisation change means may be provided. The circular to linear polarisation change means preferably comprises one or more meanderlines, or quarter-wave plates.
Polarisation change means may be provided (such as a linear polarisation in one plane to linear polarisation in another plane). The steering reflector is in one embodiment reflective for waves having a particular polarisation and transmissive for waves having a different polarisation.
The assembly is preferably adapted to receive and detect incoming radiation, but it may be an emitter. The antenna is preferably a focusing antenna.
A polarisation changer is preferably provided adjacent the antenna. The polarisation changer preferably comprises a panel of polarisation-changing material. It may serve to stiffen the antenna, or the antenna may stiffen the polarisation change material. The antenna may comprise a dish and a sheet of polarisation change material may be provided extending to the peripheral edge region of the dish.
Preferably the configuration of the assembly is such that the antenna is in front of the steerable reflector (relative to an expected direction of externally incident radiation (field of view). Preferably the antenna is in front of the detector. Preferably the steerable reflector is also in front of the detector, preferably between the antenna and the detector. Preferably the polarisation changer is between the antenna and the steering reflector. The steering reflector may be the only movable optical element.
According to a second aspect the invention comprises a field of view investigatory system having an antenna assembly in accordance with the first aspect of the invention and a signal processor, the antenna assembly having a detector which feeds signals to the signal processor.
The system may comprise a steerable interrogator system. The signal processor is preferably adapted to analyse received signals and determine whether they contain an element of a predetermined known kind.
It may be capable of interrogating a first field of view and a second field of view substantially simultaneously to determine whether the signals received by the detector from the fields of view contain an element of a known kind. A decoder may be provided which in use decodes detected signals to make available information carried by the electromagnetic radiation. There may be encryption-reading means to decipher an encrypted signal.
Protection is also sought for a vehicle provided with an antenna assembly in accordance with the first aspect of the invention or with a system in accordance with the second aspect of the invention. The vehicle may be provided with an angularly movable component and the steering reflector may be controlled at times to be at an angular position determined by the angular position of the vehicle""s angularly movable component.
The vehicle may have a first angularly moveable component and a second angularly movable component each angularly movable independently, and in which the steering reflector is controlled at times to be at a first angular position determined by the angular position if the first angularly movable component, and soon after, at substantially the same time, to be at a second angular position determined by the angular position of the second angularly movable component. The first and/or second angularly movable components may comprise an imager, such as a camera.
According to another aspect the invention comprises a method of changing the direction of the field of view of an antenna assembly, the method comprising having a stationary antenna and a movable steerable reflector, movement of the reflector moving the sensitivity response pattern of the assembly.
The method preferably comprises receiving incident radiation, but it could comprise emitting radiation to different directions.
The method may comprise either transmitting or reflecting an incident wavefront the one time (preferably the first time) that the wavefront encounters the steerable reflector, and arranging for the wavefront to encounter the steerable reflector a further time, at which time the wavefront is transmitted or reflected (the opposite of the said one time encounter with the steerable reflector). The incident wavefront may be polarised. The transmission/reflection characteristics of the steerable reflector may depend upon the polarisation state of the radiation encountering it. There may be a change in polarisation of the wavefront between its first, or said one, interaction with the steerable reflector and its further interaction with the steerable reflector.
Preferably the polarisation of the wavefront at at least one of its encounters with the steerable reflector is linear.
According to a further aspect the invention comprises either i) a method of interrogating a plurality of fields of view substantially simultaneously to determine the presence or absence of a predetermined signal, or ii) providing apparatus to achieve the method of i); the invention further comprising having a detector feed detected signals to a signal processor to perform the interrogation of the signals, and having a scanner that captures data which is fed to the detector rapidly change its field of view by providing the scanner with apparatus that is in accordance with the first aspect of the invention or which operates in accordance with any of the other aspects of the invention.
It will be appreciated that there it may be a threshold size of antenna at which it is just as easy to move the antenna as to have a smaller steerable reflector. It is believed that if the antenna is smaller than about 2 cmxc3x972 cm it may be uneconomic to use the present invention (but it may nevertheless be applied).
Preferably the antenna is at least as large as about 2 or 3 cmxc3x972 or 3 cm, or 5 cmxc3x975 cm, or 10 cmxc3x9710 cm, or 20 cmxc3x9720 cm, or 30 cmxc3x9730 cm, or even 50 cmxc3x9750 cm, or 1 meter or morexc3x971 meter or more. It should not be assumed that the antenna is necessarily square and the above size ranges could be diameters. An antenna with a size in a range extending between any of the points mentioned is envisaged. In terms of area, the antenna may have projected area of about 4 cm2, 9 cm2, 25 cm2, 50 cm2, 100 cm2, 500 cm2, 1,000 cm2, 2,500 cm2, 10,000 cm2, more, or be in a range defined by any of those points.
The mass of the antenna may be 50 g or more, or 100 g, 500 g, 1 kg, 10 kg, or more (or be between a range defined by the above values).
The steerable reflector is preferably not too big. It may be about 30 cmxc3x9730 cm (or 30 cm diameter), or less. It may have sides, or a diameter that is about 20 cm, 15 cm, 10 cm, 5 cm, 2 or 3 cm, or less, or in a range defined by any of those points. If a steerable reflector above 30 cm in size is wanted, it may be desirable to evacuate a chamber in which it resides to reduce air drag. It is possible to envisage large steerable reflectors being formed as a membrane in a frame (e.g. metalised mylar) and these could be 1 m in size.
It will be appreciated that an antenna is typically a three-dimensional structure, such as a dish, and that for any given projected area a substantially planar steerable reflector will be less massive, and therefore easier to steer.
The steerable reflector may have a projected area that is smaller than that of the antenna, possibly xc2xe of the area or xc2xd of the area, or less. The more the antenna focuses a beam the smaller the steerable reflector may be, and the closer the steerable reflector is to the focus the smaller it may be. Alternatively, the steerable reflector may have an area that is substantially the same as that of the antenna. The steerable reflector may be provided before focusing of an incident wave.
One particular embodiment of the antenna assembly is sized and adapted to operate at mm wavelengths. The invention may have an especial (but not exclusive) application at radio to sub mm wavelengths. If the wavelength gets shorter, to I.R. or optical, it can be awkward and expensive to find appropriate polarisers.
Preferably the antenna assembly operates over quite a broad bandwidth, for example 1-10 GHz (or more), most preferably at least 1 GHz, and may operate over a band of least 1 or at least 5 GHz. Preferably the assembly is adapted to operate over a bandwidth of 5-20%, or 5-10% or 15% of the central, nominal, operating frequency (for example it may have bandwidth of about 5 GHz at 30 GHz).
The speed of moving the steerable reflector from one side to the other side of its range of movement may be about {fraction (1/100)} second or less, {fraction (1/50)} second, {fraction (1/20)} second, {fraction (1/10)} second, 2 or 3 tenths of a second, half-a-second, or 1 second or more. The speed of movement may be within a range defined by any combination of the above values.
The invention could be considered to be a way of improving the speed of canning an antenna/wave-capturer from a first angle to a second angle by using a steerable mirror or reflector.
Using this in the field of mm waves is considered advantageous compared with how mm wave systems have looked to different fields of view in the past.
Having a steerable antenna that is selectively transmissive or reflective gives advantages.